Final drive assembly with differential lock

ABSTRACT

Axle-drive unit for a motor vehicle which contains a first and a second differential ( 18, 19 ) in a driven housing ( 22 ), the two differentials ( 18, 19 ) being spur-gear-type planetary gears with parallel axes, the sun wheels ( 32, 43 ) of which are in each case connected in terms of drive to the half axles ( 8, 9 ) of the first driven axle, and planet wheels ( 36, 38 ) of the two differentials ( 18, 19 ) meshing with their common ring gear ( 35 ). The housing ( 22 ) has a machined inner surface ( 50 ) which surrounds the ring gear ( 35 ) with little clearance, the ring gear is so thin in the radial direction that it is deformed in a lobe-like manner by the radial component of the tooth forces exerted by the planet wheels ( 31, 36 ). The outer circumferential surface ( 60 ) of the ring gear is placed against the inner surface ( 50 ) of the housing ( 22 ) in a manner producing friction at least locally.

BACKGROUND OF THE INVENTION

The invention involves an axle-drive unit for motor vehicles having afirst and a second driven axle which contains a first and a seconddifferential in a housing driven by an engine/transmission block, thefirst differential dividing the torque fed to it between a first halfaxle of the first driven axle and the second differential, and thelatter furthermore dividing the torque fed to it between a second halfaxle of the first driven axle and a power take-off for the second drivenaxle, the two differentials being spur-gear-type planetary gears withparallel axes, the sun wheels of which are in each case connected interms of drive to the half axles of the first driven axle, and planetwheels of the two differentials meshing with their common ring gear, theone planet carrier being connected in a rotationally fixed manner to thehousing and the other planet carrier being connected in terms of driveto the power take-off for the second driven axle.

An axle-drive unit of this type is disclosed in AT 405 923 B. In thelatter, owing to the particular design and arrangement of the twodifferentials, optimum adaptation of the moment distribution ratio isachieved with a minimum outlay on construction. Provided between thedriven housing, which contains the two differentials, and the powertake-off for the second driven axle is a fluid friction clutch as alongitudinal differential lock. The latter is not only extremely bulky,it also has the disadvantage of acting only as a lock for thedifferential between the two axles. Locking of the other differential,the differential between the two wheels of the first driven axle, is notpossible.

It is thus the object of the invention to achieve an at least limitedlocking both of the axle differential and of the longitudinaldifferential with a minimum outlay on construction.

According to the invention, this is achieved in that

a) the housing has a machined inner surface which surrounds the ringgear with little clearance,

b) the ring gear is so thin in the radial direction that it is deformedin a lobe-like manner by the radial component of the tooth forcesexerted by the planet wheels,

c) its outer circumferential surface thereby being placed against theinner surface of the housing in a manner producing friction at leastlocally, as a result of which a braking moment acts on the ring gear.

Locking thereby takes place without additional components between thehousing and ring gear. Moreover: the lock acts both on the axledifferential of the first driven axle and on the interaxle differentialbetween the two driven axles. The locking behavior also complies withthe requirements: the action occurs only at high torques, i.e. when itis actually required, but not in towing mode or during gentle cornering.Since the tooth forces between the planet wheels and ring gear areapproximately identical in both differentials, the bending stresses overthe axial length of the ring gear are also approximately the same. Inaddition, in the locked state, the housing exerts a supporting action onthe ring gear limiting the local expansion, which prevents excessivedeformation of the ring gear and tooth fractures. This is all of benefitto the service life of the ring gear.

In a preferred embodiment, the driven housing is divided in a radialplane, that part of the housing which forms the inner surface risesabove the radial plane of separation and an undercut is provided betweenthis part and the radial surface. This undercut is used for decouplingscrewing stresses caused by the bolts acting on the outside of thehousing parts and thermal stresses caused by heating of the innersurface in the locking mode. That part of the housing which forms theinner surface thereby remains dimensionally accurate. A furthercontribution to this resides in the housing part with the inner surfacehaving radial cooling ribs on its outside. This enlarges the area forthe transfer of heat to the oil or to the surrounding transmissionatmosphere.

There is great design freedom for the design of the frictionalsurfaces—both of the inner surface of the housing and of the outercircumferential surface of the ring gear. Machined, raised zones oflimited axial width have proven advantageous. The reduction in thecontact area may result in a reduction in the required contact pressurefor a certain braking action and makes it easier to keep to thefunctionally desired, exacting tolerances.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described and explained below with reference tofigures of an exemplary embodiment, in which:

FIG. 1 shows a diagrammatic view of the entire axle-drive unit,

FIG. 2 shows an axial section through the double differential,

FIG. 3 shows a radial section according to III—III in FIG. 2.

DETAILED DESCRIPTION

In the all-wheel-drive motor vehicle illustrated in FIG. 1, the engineis referred to by 1, the clutch by 2, and the manual transmission by 3.The transmission 3 ends in a driven gearwheel 4 which meshes with alarge driving gearwheel 5. The driving gearwheel 5 is already part ofthe axle-drive unit 6. The latter is adjoined by a power take-off 7 forthe rear axle drive and by a right and a left half axle 8, 9 for thedrive of the front wheels. Located in the interior of the power take-off7 is a pair of bevel gears 10, 11, and the torque for the rear axle isfed via a propeller shaft 12 to a, for example, conventionaldifferential transmission 13, in which the half axles 16, 17 of the rearwheels are driven in a known manner via a pair of bevel gears 14, 15. Afirst and a second planetary gear 18, 19 are located in the interior ofthe axle-drive unit 6 and will be described in greater detail below.

The rotating part of the axle-drive unit illustrated in FIG. 2 is,starting from the driving gearwheel 5, a driven housing 22 whichcontains the two planetary gears 18, 19. The housing 22 comprises twohousing parts 23, 24 which are jointly clamped together here to thedriving gearwheel 5 by means of threaded bolts 25. The axial position ofthe joint between the two housing parts can be established in accordancewith external requirements; either the two housing parts can have thesame depth or one of the housing parts is a flat cover and the other isa deep bell.

The first housing part 23 is at the same time the planet carrier of thefirst differential transmission 18 and is connected fixedly to thebell-shaped housing part 24 by means of the threaded bolts 25. The twotogether thus form a rigid part which is mounted rotatably in thehousing 20, 21 by means of bearings 26, 27. This first planetary gear 18also includes planet wheels 31 which can rotate about axes 30 and a sunwheel 32, which is connected by means of a wedge-shaped toothing 33 tothe left output shaft 34 to which the half axle 9 (FIG. 1) is connected.

A ring gear 35 surrounds the planet wheels 31 of the first planetarygear 18 and is at the same time also the ring gear of the secondplanetary gear 19. It meshes with the outer planet wheels 36 of thesecond planetary gear 19, which planet wheels are mounted on spindles 37which, for their part, are fastened in the second planet carrier 40. Thefirst planet wheels 36 also mesh with second planet wheels 38 which arelikewise mounted on the planet carrier 40 on spindles 39.

This second planet carrier 40 is connected via a wedge-shaped toothing41 to a hollow shaft 42 which leads into the power take-off 7 for therear axle (FIG. 1). The inner planet wheels 38 mesh with a sun wheel 43which is connected via a wedge-shaped toothing 44 to the right outputshaft 45. The latter leads via the right axle-drive shaft 8 (FIG. 1) tothe right front wheel.

The power flux runs as follows: the torque received by the large drivinggearwheel 5 is firstly divided in the first planetary gear 18 betweenthe sun wheel 32, and hence the left, front axle-drive shaft 9, on theone hand, and the hollow gear 35, on the other hand. The latterconstitutes the connection between the first and second planetary gear.The torque fed in this manner to the second planetary gear 19 is dividedvia the planet wheels 36, 38 to, on the one hand, their planet carriers40, and hence to the power take-off 7 for the rear wheels, and, on theother hand, to the sun wheel 43, and hence to the right half axle 8 ofthe front wheel drive.

According to the invention, the housing part 24 and the ring gear 35 aredesigned in a particular manner. The housing part 24 has a machined,cylindrical inner surface 50 which is extended over most of the axialwidth of the ring gear 35. It furthermore has a radial surface 51 whichlies approximately in the radial plane of separation. Threaded bolts 25are furthermore provided on the outside in flanges, for the purpose ofconnecting the two housing parts. A further radial surface 52 isprovided on the other housing part 23. The inner surface 50 extends intoa collar 53 rising above the radial surfaces 51, 52. An undercut 54 isprovided between said collar and the radial surface 51 and is used forthe purpose of keeping thermal stresses away from the radial surface 51and tensile stresses away from the inner surface 50.

FIG. 3 shows the ring gear 35 in radial section. It is dimensioned insuch a manner that it is deformed under the radial component F_(R) ofthe tooth forces F exerted by the planet wheels 31 (four in this case)to form a lobe-shaped element 35* having convexities 55* (in this casefour), this being illustrated by hatching and in a greatly exaggeratedmanner. For this purpose, the radial thickness 56 of the ring gear isselected to be of such a small size that the circumferential surface 60of the ring gear 35 is deformed outward theoretically by an amount 57 atthe points of engagement of the planet wheels 31; of course, the zones,lying in between, of the ring gear 35 are displaced inward. However,this does not occur during operation because the outwardly displacedparts of the circumferential surface 60 are previously placed againstthe inner surface 50 of the driven housing. The frictional connectionbrought about in this manner causes an at least partial locking of thetwo differentials. For the sake of completeness, it should be added thatthe planet wheels 36 of the second planetary gear 19 exert the sameaction on the ring gear 35.

The frictional connection can be optimized by the design of thecircumferential surface 60 and of the inner surface 50. For thispurpose, use can be made of suitable surface treatment processes. In theexemplary embodiment shown, again FIG. 2, the axial width which islimited by two raised zones 61 is achieved on the circumferentialsurface 50 of the ring gear. In order to improve the conduction of heat,radial cooling ribs 62 are furthermore also provided on the housing part24 and they also reinforce the housing.

1. An axle-drive unit for a motor vehicle having a first and a seconddriven axle which comprises a first and a second differential (18, 19)in a housing driven by an engine/transmission block (1), the firstdifferential (18) dividing the torque fed to it between a first halfaxle (9) of the first driven axle (8, 9) and the second differential(19), and the second differential (19) furthermore dividing the torquefed to it between a second half axle (8) of the first driven axle (8, 9)and a power take-off (7) for the second driven axle (16, 17), the twodifferentials (18, 19) being spur-gear planetary gears with parallelaxes, the sun wheels (32, 43) of which are in each case connected interms of drive to the half axles (8, 9) of the first driven axle, andplanet wheels (36, 38) of the two differentials (18, 19) meshing withtheir common ring gear (35), a first planet carrier (23) being connectedin a rotationally fixed manner to the housing and a second planetcarrier (40) being connected in terms of drive to the power take-off (7)for the second driven axle, wherein a) the housing (22) has a machinedinner surface (50) which surrounds the ring gear (35) with a clearance(63), b) the ring gear (35) is so thin in the radial direction that itis deformed in a lobe-like manner (35*) by a radial component (F_(R)) oftooth forces exerted by the planet wheels (31, 36), c) the ring gearouter circumferential surface (60) thereby being placed against theinner surface (50) of the housing (22) in a manner producing friction atleast locally, and a braking moment thereby acting on the ring gear(35).
 2. The axle-drive unit as claimed in claim 1, wherein the housingis a driven housing (22) which is divided in a radial plane into twohousing parts (23, 24), wherein that part of the housing (24) whichforms the inner surface (50) rises above the radial plane of separationand has an undercut (54) between this part and a radial surface (51). 3.The axle-drive unit as claimed in claim 2, wherein the housing part (24)with the inner surface (50) has radial cooling ribs (62) on its outside.4. The axle-drive unit as claimed in claim 1, wherein thecircumferential surface (60) of the ring gear (35) has machined, raisedzones (61) of limited axial width.